Patent GB-662137-A (GB)
Title: Automatic weighing scales
Filed: 1947-06-20
Abstract: 662,137. Electric follow-up systems. WILLIAMS, L. S., YEASTING, M. C., and TOLEDO SCALE CO. June 20, 1947. No. 16286/47. Class 40 (i). [Also in Group XVIII] A weighing scale, having a counterpoise automatically driven along a weigh-beam to counterbalance a load, is characterised by means, operated independently of the movement of the beam, repeatedly to sense and limit its displacement, and to control the flow of power to the counterpoise driving means in proportion to the sensed displacement. In Fig. I, a beam 28 fulcrummed at 3 has a poise 2 driven along it by a motor 68 to restore equilibrium when a load is applied by way of a rod 11, shelf beam 15 and a rod 22. The drive from the motor is through worm gearing 58, 59, anti-backlash gears 54 and a drum 49 connected by a pair of oppositely wound steel ribbons with the poise 2 itself and the end of a boom 37 projecting from it. The ribbons are tangential to the drum on the beam axis and so exert no moment on the beam. Control of the motor 68 is by forward and reverse spring-loaded contact plungers 83, 84, Fig. IX, carried by an insulating bracket 82 on the beam and adapted to be engaged intermittently and simultaneously by an oval cam 88 driven by a constant speed motor 89. When the beam is balanced the two contacts are engaged for equal periods so that the poise motor develops no driving torque. When the beam becomes unbalanced the contacts 83, 84 move relatively to the cam and their periods of engagement by it differ so that the motor develops torque to drive the poise in a direction to restore equilibrium, its speed progressively decreasing as equilibrium is approached. A counter 57 is also driven by the motor and indicates or records the weight. The motor 68 may, inter alia, be of the shaded pole type with two shading coils each connected across the oval cam and either contact plunger. The cam motor 89 is preferably a polarized synchronous motor to ensure correct phasing. The contact plungers may be replaced by single or paired blade contacts. These contacts fulfil the additional function of limiting the displacement of the beam under load, their unbalanced engagement by the rotating cam producing a nett force tending to restore the beam and which progressively decreases as equilibrium is restored. Return of the beam may also be accelerated by an oil dashpot 72 (Fig. I) whose plunger 73 is connected to the beam and the two sides of which communicate with gear pump 71 driven by the poise motor 68. A restoring force is applied by the dashpot to the beam which decreases with the motor speed as the beam approaches equilibrium. In the modified form of Fig. XV, the motor 155 which drives the poise 174 along the beam 164 has shading coils for forward and reverse drive controlled by contact blades 177, 178 intermittently engaged by a rotating oval cam 179 and similar to the arrangement of Figs. I and IX. Additional regulation of the motor speed is, however, obtained by a carbon pile resistance 160, adapted to be compressed by deflection of the beam and to increase the field excitation of the motor 155 with increasing beam deflection from normal. The resistance 160 is shunted by a rectifier 156 which applies a D.C. component to the motor field to brake over-running. The aim in both forms is to apply intermittently to the beam counterbalancing moments equal to and preceding pending changes due to the incremental movements of the poise. This leads to the balanced condition being reached rapidly without the poise over-running.